Ok, I just realised that I must embed in the Webhandler procedure and not the form procedure, so embedded the following in Webhandler, renamefile (before Parent):
p_web.site.UploadsPath = clip(p_web.site.WebFolderPath) & p_web.GSV('UploadsSubfolder')
....and in the setup of the form procedure I set the session variable for the uploads folder:
eg.
p_web.SSV('UploadsSubfolder','\uploads\DiscussionFiles')
This way I can have different folders for each form if I want.